The next launch window for the mission, officially known as STS-127, opens at 6:03 pm Wednesday— forecasters are calling for a 40% chance of bad weather in the vicinity of Florida's Kennedy Space Center at the time.

NASA scrubbed efforts to launch Endeavour on Saturday, Sunday, and Monday due to weather concerns. "Technically, we've been really clean the last two days with our vehicle," said NASA launch integration manager Mike Moses. "It's just been the weather scenario that got us."

STS-127 has been plagued by problems. Initially scheduled for a June launch, it was placed on hold for weeks due to hydrogen gas leaks in Endeavour's external fuel tank.

If it ever gets off the ground, STS-127 will be a 16-day mission that will see the Endeavour crew work to complete construction of Japan's Kibo space laboratory. The astronauts will add a porch-like platform to the lab's exterior that will allow experiments to be exposed to the vacuum of outer space.

Canadian Space Agency astronaut Julie Payette will be aboard the shuttle. Also part of the seven-member crew is American Timothy Kopra, who will replace Japanese astronaut Koichi Wakata—in space since March--upon arrival at the ISS.

NASA is expected to phase out the space shuttle program starting next year. Plans call for the development of an Apollo-style rocket and capsule system, dubbed Ares and Orion, to replace the orbiter.

Obama administration officials, however, have recently raised questions about the plan's cost and practicality.

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